Engine components having the ability for longer life under operational conditions is important.
This invention relates to providing components wherein the components operate under high stress conditions. Such components may be for use in engines. In particular, the invention is directed to providing and repairing blades for steam turbines.
In the operational conditions of steam turbines, particular locations in the blades can be subjected to increased stresses caused by high pressure, high temperature, or high corrosion effects, stress concentration, dynamic loading or a combination of them.
The base material of blades is normally of a hardened steel. Portions of the blade which are subjected to increased stress can be provided with components of different characteristics. For instance, a Stellite shield may be provided to a particular portion of the foil vulnerable to particular erosive conditions. Additionally, a wire can be provided to stiffen a blade to reduce bending moments, introduce damping, increase blade frequency and allow for minor tuning adjustments. The wire passes through a wire hole in the blade. Cracking in these holes sometimes is traceable to stress corrosion cracking due to overheating during the brazing process of the wire into the wire hole. Sometimes cracking is the result of pitting and/or corrosion fatigue at the corner of the wire hole.
In other cases, lashing lugs are forged as part of the blade foil which, when assembled on the wheel, are then welded together to form a lashing wire. High hardness in the heat affected zone caused by the welding also provides a location prone to stress corrosion cracking.
Because of the lower stress at the fusion location of the lashing lugs, it has been common practice to use a non-hardenable filler material for the lug welds. Also, it has been common practice that the tie wire material be made of non-hardenable material. Thus, the non-hardenable wire material and non-hardenable filler metals are not stress corrosion sensitive due to the low hardness. However, wherever non-hardenable filler metal is welded to hardenable metal, there will be a heat affected zone in the hardenable material.
A heat affected zone is where the heat of the welding has altered or affected the hardenable material. There is a higher and sometimes a lower than normal hardness at that location, in close proximity. Since a higher hardness produces a higher strength, one would think this condition would be desirable. However, these heat affected zones are subject to the high risk of stress corrosion cracking. As such, these locations in a steam turbine blade are subject to premature failure.
The present invention seeks to overcome this disadvantage.